<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>demo</title>
<style>
body,h1,h2,h3,h4,h5,h6,hr,p,blockquote,dl,dt,dd,ul,ol,li,pre,form,fieldset,legend,button,input,textarea,th,td{margin:0;padding:0;}
body,button,input,select,textarea{font:12px/1.5 tahoma,arial,\5b8b\4f53;}
h1,h2,h3,h4,h5,h6{font-size:100%;}
address,cite,dfn,em,var{font-style:normal;}
code,kbd,pre,samp{font-family:courier new,courier,monospace;}
small{font-size:12px;}
ul,ol{list-style:none;}
a{text-decoration:none;}
a:hover{text-decoration:underline;}
sup{vertical-align:text-top;}
sub{vertical-align:text-bottom;}
legend{color:#000;}
fieldset,img{border:0;}
button,input,select,textarea{font-size:100%;}
table{border-collapse:collapse;border-spacing:0;}
.clear{clear: both;float: none;height: 0;overflow: hidden;}
p{font-size: 100px;}
#mask{
 background: #000;
 opacity: 0.75;
 filter: alpha(opacity=75);
 height: 1000px;
 width: 100%;
 position: absolute;
 left: 0;
 top: 0;
 z-index: 1000;
}
#login{
 position: fixed;
 left: 30%;
 top: 30%;
 z-index:1001; 
}
.loginCon{
 width: 670px;
 height: 380px;
 background: #fff;
 border:5px solid #F01400;
}
#close{
 width: 30px;
 height: 30px;
 background: blue;
 cursor: pointer;
 position: absolute;
 right: 10px;
 top: 10px;
}
#btnLogin{
 width: 80px;
 height: 40px;
 background: #F01400;
 margin:0 auto;
 display: block;
 cursor: pointer;
 border-style: none;
 color: #fff;
 font-size: 16px;
}
</style> 
</head> 
<body>
<button id="btnLogin">登录</button> 
<!--
<div id="mask"></div>
<div id="login">
 <div class="loginCon">
 <div id="close"></div>
 </div>
</div>
-->
<p>1</p>
<p>1</p>
<p>1</p>
<p>1</p>
<p>1</p>
<p>1</p>
<p>1</p>
<p>1</p>
<p>1</p>
<p>1</p>
<script type="text/javascript">
function openNew(){
 //获取页面body！内容！的高度和宽度
 var sHeight=document.documentElement.scrollHeight;
 var sWidth=document.documentElement.scrollWidth;
 //获取可视区域高度，宽度与页面内容的宽度一样
 var wHeight=document.documentElement.clientHeight;
 //创建遮罩层div并插入body
 var oMask=document.createElement("div");
 oMask.id="mask";
 oMask.style.height=sHeight+"px";
 //宽度直接用100%在样式里
 document.body.appendChild(oMask);
 //创建登录层div并插入body
 var oLogin=document.createElement("div");
 oLogin.id="login";
 oLogin.innerHTML="<div class='loginCon'><div id='close'></div></div>"
 document.body.appendChild(oLogin);
 //获取login的宽度和高度并设置偏移值
 var dHeight=oLogin.offsetHeight;
 var dWidth=oLogin.offsetWidth;
 oLogin.style.left=(sWidth-dWidth)/2+"px";
 oLogin.style.top=(wHeight-dHeight)/2+"px";
 //获取关闭按钮
 var oClose=document.getElementById("close");
 oMask.onclick=oClose.onclick=function(){
  document.body.removeChild(oMask);
  document.body.removeChild(oLogin);
 }
}
window.onload=function(){
 var oBtn=document.getElementById("btnLogin");
 oBtn.onclick=function(){
  openNew();
 }
}
</script>
</body>
</html>